The present invention relates to the field of co-fired ceramic structures and, more particularly, to techniques for processing such structures.
Glass ceramic structures, usually and preferably multilayered, are used in the production of electronic substrates and devices. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual glass ceramic layers formed prior to lamination, which, upon sintering, will become a sintered dense metal interconnection of metal-based conductor.
In general, conventional ceramic structures are formed from ceramic green sheets which are prepared by mixing a ceramic particulate, a catalyst (e.g., such as that disclosed in Herron et al. U.S. Pat. No. 4,627,160) a thermoplastic polymer binder, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After blanking, stacking and laminating, the green sheets are eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate.
The electrical conductors used in formation of the electronic substrate may be high melting point metals such as molybdenum and tungsten or a noble metal such as gold. However, it is more desirable to use a conductor having a low electrical resistance and low cost, such as copper and alloys thereof.
Use of copper-based conductors in the multilayered structures requires the use of process techniques which do not oxidize the copper during the removal of binder resins and solvents, and sintering of the ceramic particulates together into the densified ceramic substrate.
For example, a typical firing cycle consists of burning the binder off in a steam ambient, typically water vapor plus hydrogen, and then replacing the steam ambient with an inert (neutral) ambient such as nitrogen and sintering the structure to its final densified state, followed by a cool down, again in an inert atmosphere such as nitrogen.
This seemingly simple firing cycle is, in fact, extraordinarily complex in nature and has taken years and large expense to achieve. The very fact that there are numerous patents in this area indicates the delicacy of the technology. Ambients, material sets and processing parameters cannot be considered to be interchangeable in this art. Those skilled in the art are painfully aware of the need to control a myriad of parameters in order to attain a suitable multilayer metallized part. Although the representative art does contain a variety of teachings with respect to materials and processes, substitution is not at all obvious in this field of endeavor. Rather, extensive experimentation is necessary in order to modify any teachings in this field. Is is, in fact, frequently necessary to invest substantial experimental effort in order to duplicate well-documented and patented techniques, let alone seek to modify facets of those techniques.
It is not an understatement to say that improvements in this art come in small steps rather than in great leaps.
The starting point for our discussion is Herron et al. U.S. Pat. No. 4,234,367, which is assigned to the assignee of the present invention. There, the basic firing cycle for glass ceramic structures is set forth. An initial preheating step in nitrogen is followed by binder burnoff in a steam ambient consisting of hydrogen and water vapor. The ratio of the hydrogen to the water vapor is precisely controlled so as to cause oxidation of the pyrolyzed binder residues, i.e. carbon, to carbon dioxide gas without causing oxidation of the copper. Oxidation of the copper, of course, is to be avoided since it is accompanied by a volumetric change which can be disastrous to the integrity of the glass ceramic structure. After binder burnoff, the atmosphere is changed to nitrogen and the temperature is raised to accomplish sintering of the structure wherein densification and coalescence takes place. Cooldown follows in the same nitrogen atmosphere. Note that Herron et al. prefers a neutral ambient for sintering since reducing ambients can occasion adhesion problems.
Kamehara et al. U.S. Pat. No. 4,504,339 discloses a firing cycle for a glass ceramic substrate wherein the preheating and binder burnout steps take place in an inert (neutral) atmosphere containing water vapor and the densification and coalescence of the glass ceramic particles takes place in an inert atmosphere.
Takeuchi et al. U.S. Pat. No. 4,649,125 discloses a series of firing atmospheres for a multilayer ceramic structure, each of which does not oxidize to any appreciable extent the metal conductor material, e.g. copper. The atmospheres may be nitrogen, nitrogen plus oxygen, nitrogen plus hydrogen, or hydrogen plus nitrogen plus water vapor. There is no indication where in the firing cycle these atmospheres are to be utilized or if they are to be utilized throughout the entire firing cycle. Such a limited teaching provides little or no guidance to one trying to improve the state of the art.
Bezama et al. U.S. patent application Ser. No. 07/103,768, filed Nov. 23, 1987, abandoned, assigned to the assignee of the present invention, discovered that in the firing cycle of Herron et al discussed above, there is a certain amount of residual carbon left after binder burnoff which can lead to problems later on during coalescence such as porosity due to the evolution of carbon dioxide gases. Accordingly, Bezama et al. inserted a drying step between binder burnoff and coalescence which suppresses the further oxidation of any residual carbon. The improvement of Bezama et al. has been found to be effective in reducing the amount of porosity in the glass ceramic body.
Chance et al. U.S. patent application Ser. No. 07/418,435, filed Oct. 6, 1989, assigned to the assignee of the present invention, recognized that notwithstanding the previous efforts of others to improve the firing of glass ceramic substrates, there still remained the problem that the metallic vias did not completely seal to the glass ceramic body after sintering. The resulting gaps and cracks were subsequently "backfilled" or filled with a polymer or other sealing material after sintering, as taught for example in Acocella et al. U.S. patent application Ser. No. 07/503,495, filed Mar. 30, 1990, assigned to the assignee of the present invention. What Chance et al. proposed was to first, fire each multilayer ceramic structure generally according to the teachings of Herron et al. and then, pressure sinter in a nitrogen-rich atmosphere each glass ceramic structure in a special fixture.
While the pressure sintering of Chance et al. has worked well in practice, there are two main problems of a practical nature that need to be overcome. The first is that throughput is slow because of the need to perform a separate pressure sintering step for each glass ceramic structure and only one glass ceramic structure can fit in any one fixture at a time. The second is that the firing process is lengthened because of the pressure sintering step, thereby requiring longer processing times and extra expense.
It is apparent, therefore, that notwithstanding the prior efforts and improvements of others, there still remains a real need to improve the firing cycle for glass ceramic structures.
Accordingly, it is an object of the present invention to have an improved firing cycle for glass ceramic structures.
It is another object of the present invention to have an improved firing cycle without otherwise adversely impacting on throughput and processing time.
It is yet another object of the present invention to have an improved firing cycle which produces a glass ceramic structure having sealed vias without the need for further processing operations such as pressure sintering and backfill.
These and other objects of the invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.